F1technical.net

Diesel makes colder exhaust.

marekk wrote:

ringo wrote:Where did you get that 1 bar value from?

atmospheric pressure.

If it was just over 1 bar, a turbo charger would not work on any engine. Turbos work on pressure difference.
so the pressure directly from the exhaust port is not 1 bar.
This pressure is representative right as exhaust stroke starts.

1 Bar at pipe's exit - please read a little bit more carefully.

As the exhaust stroke starts, there is just 1 bar inside - exhaust valves open long befor BDC.

You have to decide - exhaust pressure or speed. You cant have both at the same time in subsonic flow.

Explain also why BMW v8 exhaust is 950 degrees.
Is their engine burning 3 times hotter?

It's temp of exhaust on exhaust port. And not 3 times hotter - it needs to have just +33% energy.

This lemans, and we know diesels burn cooler.

Actually flame temp of diesel is slightly higher than for light fuel.

Correct, flame temp of diesel is higher and but for a shorter duration. Modern commo rail diesels utilise more than one injection event to maintain flame speed and combustion duration. the turbo has nothing to do with it. Diesel goes boom when a certian level of activation energy is reached. Turbo's allow a shorter stroke to achieve the desired activation energy (pressure and temperature). A shorter stroke allows better drivability and higher rpm (Power)

Diesels do not ever run cooler than a petrol/gasoline engine in an automotive application.
Diesel fuel carries a higher heating value, a more intensive combustion (detonation) and higher friction due to higher peak and average pressures.

Hence temperature at the exhaust valve will be close to the metallurgical limit but drops from there to the exit.

Someone refered to teh exhaust gase flow as a series of pulses. that is true and correct but irrelevant. Many of the pictures shown thus far that illustrates exhaust seesm to neglect that the visible components of the exhaust is solid matter and those particles have mass and follow a trajectory. he gas does not not follow the path of the solids(solids being unburnt fuel, oil, bits of engine etc)

n smikle wrote:Diesel makes colder exhaust.

at exit due to the excess air creating NOx which has a cooling effect.
at the exhaust valve the tmeperature is higher than for gasoline/petrol

n smikle wrote:[youtube]http://www.youtube.com/watch?v=B0Pnvgc2_gA[/youtube]

I think the mass flwo there is SIGNIFICANTLY higher than a 2.4L v8 F1 engine

diesels have colder exhuast. It's constant pressure combustion. Gasoline is constant volume combustion.

So many trying to prove me wrong with no foundation to their opinion, and keep missing the target.

Raptor22 wrote:

n smikle wrote:Diesel makes colder exhaust.

at exit due to the excess air creating NOx which has a cooling effect.
at the exhaust valve the tmeperature is higher than for gasoline/petrol

the exhaust is colder full stop.

That reason you stated was desperate attempt to validate your claim.

I suggest dropping diesel vs gasoline peak temperature discussion and focus on finding an agreement on exhaust conditions estimate.

We have fixed:
-mass flow rate aroun 0.20kg/s per bank
-energy content around 300kW per bank

We have to converge on:
-2 out of 3 between temperature, pressure and density at output section
-reasonable size of exhaust

Then we can move on to discuss exhaust gas behaviour outside

Raptor22 wrote:I think the mass flwo there is SIGNIFICANTLY higher than a 2.4L v8 F1 engine

Mere 2,000 times. For one CF6-80.

shelly wrote:I suggest dropping diesel vs gasoline peak temperature discussion and focus on finding an agreement on exhaust conditions estimate.

We have fixed:
-mass flow rate aroun 0.20kg/s per bank
-energy content around 300kW per bank

We have to converge on:
-2 out of 3 between temperature, pressure and density at output section
-reasonable size of exhaust

Then we can move on to discuss exhaust gas behaviour outside

It was the easy part.

Can't find good pictures for 100 degree bend like in R31, but this one shoul give us an idea:


Renault has built-in separation wall, so we deal basically with 2 separate flows (speed difference about 2 times), and we should discuss both flows.
I think slower one goes under floor and to the back, quicker flow goes sideways.
Mass flow/energy proportion should be about 2:1, so we have 100kW under the floor and 200 kW to the side. How much of this 100kW is transfered to tarmac/floor and radiated before reaching diffuser exit ? Hard to guess, we need to investigate further.

Why is one slower by half?

Using that diagram light orange is 12 red is 15. 12/15 is not a half.

Secondly we do not know the nature of the bend or pipe diameter in the bend. It's simpler to assume the speed is the same.

mass flow is not what you say. Both still have equal mass flow.

ringo wrote:Why is one slower by half?

Using that diagram light orange is 12 red is 15. 12/15 is not a half.

Secondly we do not know the nature of the bend or pipe diameter in the bend. It's simpler to assume the speed is the same.

mass flow is not what you say. Both still have equal mass flow.

Green is 6, red is > 12. mass flow at constant density is proportional to speed.
It's simpler to not post if you have nothing to say.

The pipe doesn't end in the bend.

Look on the exit.

mass flow is unchanging, it's not proportional to anything.

If 0.1 kg/s goes either side. 0.1 kg/s passes out per side.

I'll shut up now until sunday's race.

I like at 1:56 how the dirt shows boundary layer starts to from over the car before it gets lifted.

@marekk: I think we need an estimate of exit conditions before thinkinkg of exit path.
The picture you show is interesting; however as far as the thin wall in the middle of exhaust pipe is concerned, I am not sure that it is there for splitting flow: I have seen it on more than one f1 car in the 80s-90s and it can be ralated to structural resons of the pipe itself.

Anyway: do we all agree that exaust speed is 100+ m/s?

shelly wrote:@marekk: I think we need an estimate of exit conditions before thinkinkg of exit path.
The picture you show is interesting; however as far as the thin wall in the middle of exhaust pipe is concerned, I am not sure that it is there for splitting flow: I have seen it on more than one f1 car in the 80s-90s and it can be ralated to structural resons of the pipe itself.

Anyway: do we all agree that exaust speed is 100+ m/s?

Real flow is much more complex than this pictured one (not steady, much slower in boundary to pipe walls ...), and we don't know anything about pipes geometry and inner build jet, but it gives us an overview.

OK. We need 2 estimations:
1. Exhaust exit area
2. Exhaust exit temp

Until we see better pictures (i.e. exhaust glowing in daylight) i go for 0,01 m^2 (113 mm diameter) and 900K, this will give mean exit speed of 53,85 m/s (you can use volumetric or mass flow calculation method, results are still the same).

I agree inner wall may play structural role (they'll need to precisely control geometry), but it will split and straighten flows to.

On this picture exit looks asymmetrical, with bigger inner camber and flattened bottom: